1. Field of the Invention
The present invention relates to a salt-soluble ceramic fiber composition for a high-temperature thermal insulator, and, more particularly, to a ceramic fiber composition for a high-temperature thermal insulator having excellent salt-solubility, wherein the fiber composition includes SiO2 as a network-forming oxide, CaO as a network-modifying oxide, MgO, Al2O3 as an intermediate oxide, B2O3 serving as both a flux and a network oxide in the composition, Na2O serving as a flux, and K2O at a proper ratio so as to prepare a composition having improved solubility of a fiber composition in an artificial saline body fluid. Also, the present invention relates to a composition in which a content of the sum of B2O3 and Na2O+K2O present as the flux is controlled to show constant high-temperature heat resistance, and which has improved solubility (biodegradability) in a saline body fluid compared to a conventional salt-soluble ceramic fiber composition and an increased production yield in a high-temperature production process.
2. Discussion of Related Art
Ceramic fibers have been used as materials such as heat-insulating materials, cold-insulating materials, thermal insulators, soundproof materials, sound-absorbing materials and filtering materials because of their low thermal conductivity and long and thin shape.
The term “fiber for a fire-resistant thermal insulator” used as a thermal insulator generally refers to a fire-resistant fiber that can be used at a temperature of 600° C. or higher at which conventional mineral wool is used. Fibrous blanket thermal insulators used at a high temperature are classified into 5 types: type 1 (732° C.) to type 5 (1,649° C.) according to the ASTM C982, based on the thermal contraction coefficients measured at a high temperature. The “safe use temperature” of a conventional fiber is defined as a temperature at which a fiber has a linear thermal contraction coefficient of 5% or less when maintained at a corresponding temperature for 24 hours.
In recent years, the most widely used fiber for a fire-resistant thermal insulator is an Al2O3—SiO2 (RCF-AS)-based fiber, the safe use temperature of which is in a range of 1100 to 1260° C. Conventional known techniques associated with the Al2O3—SiO2-based fiber are as follows. U.S. Pat. Nos. 2,873,197 and 4,555,492 disclose an Al2O3—SiO2—ZrO2 (RCF-ASZ)-based fiber prepared by adding a certain amount of a ZrO2 ingredient to an Al2O3—SiO2-based composition, wherein a safe use temperature of the fiber increases to 1430° C.
U.S. Pat. No. 4,055,434 discloses a fiber composition prepared by adding up to 16% burned dolomite to an Al2O3—SiO2-based composition as CaO and MgO sources, wherein the fiber has a heat-resistant temperature of 760 to 1100° C. U.S. Pat. No. 3,687,850 discloses that a silica fiber including 76 to 90% SiO2 and 4 to 8% Al2O3 has a heat resistance of 1093° C. without precipitation of crystals, wherein the silica fiber is prepared by adding an acid to a fiber composition composed of SiO2, Al2O3, R2O, RO and B2O3 and dissolving the RO, R2O and B2O3 ingredients. However, although the heat resistance and dissolution characteristics in acid are considered in order to prepare the conventional fiber for a fire-resistant thermal insulator, there is no consideration of the dissolution characteristics in a saline solution such as a synthetic body fluid. Also, low solubility in a physiological medium may be caused due to high Al2O3 content (i.e., 4% or more).
Recently reported data shows that a fiber having low solubility in a physiological medium is inhaled in a finely ground fiber shape and accumulated in the lungs, which leads to damage to the human body. Therefore, there has been ardent research to develop an inorganic fiber composition so as to increase solubility in a physiological medium to minimize possibility of being harmful to the human body and to satisfy high-temperature physical properties as well.
A glass fiber composition easily dissolved in the physiological medium is known as follows. For example, there are a bioabsorbable glass fiber composition including CaF2, ZnO, SrO, Na2O, K2O and Li2O in addition to CaO and P2O5 (U.S. Pat. No. 4,604,097), a fiber composition obtained by adding P2O5 to a conventional soda lime borosilicate glass fiber composition (International Patent WO92/0781), a fiber composition obtained by adding an increased amount of B2O3 and other Na2O to a soda lime borosilicate composition (U.S. Pat. No. 5,055,428), etc. However, these compositions have a disadvantage in that their heat resistance is low because they are composed of composition regions, each of which includes a relatively high content of an R2O ingredient, there is no mention of the safe use temperature or they are only actually used as a thermal insulator at 350° C. or lower in buildings, and there is a limitation to use as a biodegradable material that can be used at a high temperature.
Also, examples of the fiber composition having excellent solubility in a synthetic body fluid, which can be used as a fire-resistant fiber at a high temperature, are listed as follows. For example, there are a modified fiber composition having improved solubility in a synthetic body fluid and enhanced fire resistance by reducing a content of Al2O3 and increasing a content of MgO in a conventional mineral wool including components such as CaO, MgO, SiO2 and Al2O3 (International Patent WO87/05007), a fiber composition obtained by selectively adding components such as MgO, alkali oxide, Al2O3, ZrO2, B2O3 and Fe2O3 to SiO2 and CaO (International Patent WO89/12032), a fiber composition having a use temperature of 800° C. to 1000° C. by reducing a content of Al2O3 while maintaining contents of SiO2, CaO and MgO (International Patent WO93/15028), etc. However, these compositions may only be used in limited fields in which their maximum safe use temperature is defined as 815° C. to 1000° C. (a linear thermal contraction coefficient of 5% or less when maintained for 24 hours). Also, since the above-described fiber compositions do not include a flux ingredient, it is difficult to avoid deterioration of performances such as production yield and biodegradability.
Also, examples of the fiber composition having a maximum safe use temperature of 1260° C. and showing excellent solubility in a synthetic body fluid are as follows. International Patent WO94/15883 discloses a fiber composition region in which the remaining SiO2 content accounts for 21.8 mol % or more by adding Al2O3 and ZrO2 to SiO2, CaO and MgO, but it is difficult or impossible to form a fiber in the composition regions having high SiO2 contents of 70.04 mol %, 73.09 mol % and 78.07 mol % (high contents of non-fibrous materials). International Patent WO97/16386 discloses a biodegradable fiber composition having a linear thermal contraction coefficient at 1,260° C. of 4.5% that is easily formed into fiber, wherein a composition region having a high content of SiO2 includes MgO and SiO2 as major ingredients, has a CaO content of 1% or less and includes 0 to 2% of Al2O3, ZrO2 and B2O3, which are added as other viscosity modifiers. However, a fiber product having such a composition region has high thermal conductivity due to thick average fiber size and a relatively high linear thermal contraction coefficient at a safe use temperature (3% or more). Because an excessive content of SiO2 is used to increase the safe use temperature, the fiber compositions have much lower biodegradability than the fiber composition prepared according to the present invention, and their production yield is decreased, a large amount of dust is generated upon fiber production, and product qualities such as a tensile strength may be degraded.
Representative examples of the currently developed ceramic fiber compositions have been described above. The desired physical properties of the ceramic fiber compositions are listed based on the above-mentioned techniques known in the art, as follows.
A method of forming a ceramic fiber composition into a fiber includes a blowing process of forming a fiber using compressed air or a compressed steam and a spinning process of forming a fiber by dropping a molten material on a cylinder that is rotating at a high speed. An ideal viscosity of the fiber composition which is suitable to form a fiber using the spinning or blowing process should be low, for example, in a range of 20 to 100 poises, or be similar to or not highly different from that of a conventional SiO2—Al2O3-based composition. When the viscosity at a fiber-forming temperature is too high, a diameter of the fiber is increased, which leads to generation of a large amount of a thick non-fibrous material (shot). On the other hand, when the viscosity is excessively low, a fiber becomes short and thin which leads to generation of a large amount of a fine non-fibrous material (shot). In general, since the viscosity of a glass melt solution depends on glass compositions and temperatures, the compositions should be properly designed to maintain a suitable fiber-forming viscosity. Also, since high-viscosity compositions need to be formed into fiber at a higher temperature, their viscosity should be controlled around a fiber-forming temperature.
Also, a ceramic fiber used for high-temperature insulation should have high thermal resistance and also show excellent durability even when thermal stress is repeatedly applied to a furnace material. Therefore, even when the ceramic fiber is exposed to heat corresponding to the use temperature, its physical properties should be hardly changed. The use temperature of the ceramic fiber is associated with contraction at the use temperature.
The contraction of a fiber product is affected by the viscosity of glassy fiber composition at a high temperature, the kind and amount of crystals generated and growing when exposed to heat upon use of a product, the crystal precipitation temperature and the high-temperature viscosity of a glassy phase remaining after crystal precipitation. Since crystals precipitated at a high temperature have a higher specific gravity than a conventional glassy fiber, stress is caused in a crystal interface due to the crystal precipitation and growth, and thus fibers are cut or deformed due to the stress, which leads to contraction of the fiber. When the fiber is present as a glassy phase without precipitation of crystals at a high temperature, the viscosity of the fiber like glass is also gradually decreased at a relatively low temperature, which results in an increase in contraction of the fiber. Also, even when the glassy phase remaining after the crystal precipitation has a low high-temperature viscosity, the fiber contraction is increased due to liquid phase sintering and deformation caused by the viscous flow. A fiber prepared from a composition having a low contraction rate at a high temperature should have a suitable crystal precipitation quantity and rate and a proper precipitation temperature. Also, the solubility of the ceramic fiber in a synthetic body fluid should be hardly changed even when the ceramic fiber is exposed to a high temperature. Therefore, it is very important to select a composition which has high solubility in a synthetic body fluid, is easily melted and formed into fiber, and has a low linear thermal contraction coefficient at a high temperature.
Furthermore, a glass wool, a mineral wool and a ceramic fiber has excellent solubility in a synthetic body fluid compared to an asbestos fiber which has been known as a carcinogenic substance, but have not been proved to be harmful to human beings. The toxicological test results using animal tests indicate that the solubility of a fiber in a synthetic body fluid is particularly correlated with harmfulness in the animal tests. However, it was reported that a fiber having a dissolution rate constant (Kdis) of 100 ng/cm2·hr or more does not develop fibrosis or tumor in an animal inhalation test (Inhalation Toxicology, 12:26 to 280, 2000, Estimating in vitro glass fiber dissolution rate from composition, Walter Eastes). Dissolution rate constants (Kdis) of currently developed biodegradable fibers are in a range of 300 to 600 ng/cm2·hr. However, the present invention aims to provide a fiber composition capable of minimizing harmfulness to the human body compared to conventional biodegradable ceramic fibers by setting a reference value of the solubility of a ceramic fiber composition in a synthetic body fluid to 700 ng/cm2·hr or more.